1. Field of the Invention
This invention relates to a turbo-molecular pump, a substrate processing apparatus, and a method for suppressing attachment of depositions to the turbo-molecular pump, and in particular to a turbo-molecular pump for discharging a deposition-causing gas.
2. Description of the Related Art
Generally, substrate processing apparatus for performing plasma processing on wafers for semiconductor devices is equipped with a processing chamber (hereinafter referred to as “chamber”) for performing a predetermined processing on substrates housed therein. When the plasma processing in the substrate processing apparatus is etching processing, CF gas or like is introduced into the chamber as a processing gas, and the plasma generated from the CF gas is used in the etching processing. During the etching processing, the plasma reacts with an etching film of the wafers, but a certain amount of unreacted processing gas is left over. Also, reaction products are sometimes generated by the reaction between the plasma and the etching film, and the generated reaction products may vaporize and mix with the unreacted processing gas.
After the etching processing, the unreacted processing gas alone or the mix of the unreacted processing gas and the vaporized reaction product (hereinafter both referred to as “deposition-causing gas”) is discharged from the chamber using a turbo-molecular pump connected to the chamber.
FIG. 8 is cross-sectional view schematically showing a construction of the conventional turbo-molecular pump. Note that in FIG. 8 the exhaust gas from the chamber is flowing from an upper part to a lower part.
In FIG. 8, a turbo-molecular pump 80 includes a rotor 81 which is a rotating body having a rotor shaft extending in a direction of flow of the exhaust gas and a cylindrical case 82 which houses the rotor 81. A plurality of blade-form rotor blades 83 project orthogonally from the rotor 81 with respect to the rotor shaft. A plurality of blade-form stator blades 84 project orthogonally with respect to the rotor shaft from an internal wall of the case 82 towards the rotor 81. The rotor blades 83 and the stator blades 84 form a turbine. When the rotor 81 is rotated at high speed by an induction motor, the turbo-molecular pump 80 drives gas that is upstream of the turbine to a downstream side of the turbine at high speed. The gas driven to the downstream side of the turbine is discharged to an external portion via an exhaust pipe 85.
Note that gas is continuously fed by the turbine into a space LS (herein after referred to as a “lower space”) between the turbine and the exhaust pipe 85 in the turbo-molecular pump 80. Also, since a volume of the exhaust pipe 85 is smaller than a volume of the lower space LS, gas fed into the lower space LS by the turbine builds up in the lower space LS. As a consequence, the pressure rises in the lower space LS. Here, the gas discharged by the turbo-molecular pump 80 is a deposition-causing gas. Thus, if the pressure in the lower space LS exceeds the saturated vapor pressure of the processing gas or the like in the deposition-causing gas, components of the processing gas liquefy and the reaction product solidifies, causing attachment of depositions to surfaces of the rotor 81 or the like which face the lower space LS. Since the depositions lower the discharge efficiency of the turbo-molecular pump 80, and in a worst-case scenario, causes the turbo-molecular pump 80 to seize, there is a need to suppress the depositions.
In the prior art, a turbo-molecular pump including a heating apparatus which generates eddy currents using the rotation in the turbo-molecular pump has been developed as a turbo-molecular pump for suppressing attachment of depositions. In this turbo-molecular pump, the Joule heat caused by the generated eddy currents is transmitted to the various component parts thereof (see Japanese Laid-Open Patent Publication (Kokai) No. H9-32794, for instance). In such turbo-molecular pumps, the temperature of the component parts is raised to vaporize reaction products or processing gas which have reached the surface of the component parts, thereby suppressing attachment of the depositions.
However, the following problems occur when raising the temperature of the component parts by transmitting heat from the heating apparatus.
1. Since heat is applied to all the component parts, the degree of thermal expansion of the component parts increases. Such thermal expansion may damage the component parts.
2. The depositions temporarily attaching to the component parts forms an insulating layer, and it is not therefore possible to raise the temperature at the surface of the depositions. As a result, new depositions attaching to the surface of the depositions cannot be suppressed.3. Since it is not possible to raise the temperature of component parts not mechanically connected to the heating apparatus, attachment of depositions cannot be suppressed in all of the component parts.
In other words, attachment of depositions to the component parts cannot be reliably suppressed by transmission of heat from the heating apparatus.
Also, since the rate of attachment of depositions varies depending on the processing gas, it is difficult, when attachment of the depositions to the component parts cannot be reliably suppressed, to set a timing at which the turbo-molecular pump are to be replaced. Hence, stable running of the turbo-molecular pump and, by extension, the substrate processing apparatus cannot be ensured.